if-let/if-let*/and-let/..

[Stefan Monnier]

This if/when/and-let/let* business looks pretty messy to me:

- why do we have and-let* ?  According to its docstring the only case
  when it differs from when-let* is when body is nil, but in that case
  you're better off moving the last binding to the body (and use
  when-let*).

- why do we have foo-let vs foo-let* ?  The fact that each stp is depend
  on the previous means that a "parallel let" semantics doesn't make
  much sense, so only the "let*" semantics makes sense and we don't want
  to have both of them.

Currently, almost all packages outside of Emacs which use those things
suffer from annoying obsolescence warnings which don't seem to be worth
the trouble.


        Stefan